Bakery product characteristics as influenced by convection heat flux

Energy consumption and product quality changes are often observed as the ratio of the convection to the conduction modes of heat transfer varies in industrial baking ovens. Air and oven-wall temperature profiles as well as air velocity can affect the convection/radiation heat transfer and hence the quality of the baked products. A programmable pilot-plant oven was used to establish five baking profiles by measuring the total heat flux and the convective component using a special heat flux meter called an h-monitor. The purpose was to keep the total heat flux delivered to the h-monitor constant while varying the convective component from 27% (for the standard profile) to 11%, 22%, 33% and 37% by modifying air characteristics and wall temperatures. Industrial cupcakes were baked using the five established baking profiles and then evaluated in terms of quality parameters. Compared to the standard profile, a 10% reduction in volume expansion and a 30% increase in texture properties were observed for extreme oven conditions; top colour was always darker but more uniform for the conditions with less convection. The moisture content of the middle part of the cake was always higher than that of the top, bottom and sides. Baking industries are interested in using the pilot-scale oven to modify baking profiles for the purposes of quality improvement, product development and energy savings, rather than having to engage in high-cost trial and error practices on the production site.     

A NUMERICAL APPROACH WITH VARIABLE TEMPERATURE BOUNDARY CONDITIONS TO DETERMINE THE EFFECTIVE HEAT TRANSFER COEFFICIENT VALUES DURING BAKING OF COOKIES

The increasing trade of ready‐to‐eat foods such as cookies highlights an interest in quality defects during baking. Heat (h and thermal diffusivity) and mass (mass transfer and diffusion coefficients) transfer parameters are significant parameters affecting the quality changes. Therefore, it is important to determine these parameters for modeling and process optimization studies. Among these, the h is important, revealing the relationship between the heating medium and product surface. As baking involves a simultaneous heat and mass transfer involving moisture diffusion and heat conduction inside and convective heat and mass transfer outside, a lumped system method may not be an accurate choice to determine the h value. Changes in the product volume and contact heating from bottom of the product also bring extra challenges to the determination of h. Therefore, the objective of this study was to use realistic approaches including simultaneous heat and mass transfer to determine the changes in h. The h_effvalues for the bottom and top surface of the cookies were then determined, applying a numerical procedure where the surface temperature changes were the boundary conditions with evaporation on the surface. The h_band h_t values increased with baking temperature and varied with baking time. The results of this study showed that evaporative mass flux for the top surface, heat flux for the bottom surface and the product's volume changes were significant in the variation of h values.     

Heat and Mass Transfer Coefficients in Drying of Starch Based Food Systems

Experiments were conducted to measure the temperature and moisture content of starch and starch-fructose samples during drying. A two-dimensional finite element model was developed. An exhaustive parameter estimation technique was applied in conjunction with the model and drying data to estimate the moisture filtration coefficient k_p and the surface heat and mass transfer coefficients, h_q and h_m. Minimization of the deviation between experimental and simulation data resulted in a k_p value of 0.51*10^?7 kg.m/N.h and 0.52*10^?7 kg.m/N.h for the starch and starch-fructose samples. Corresponding values of h_q were 15.8 W/m²K and 21.2 W/m²K and that of h_m were 1.12*10^?6 m/s and 0.94*10^?6 m/s respectively.     

Heat transfer coefficients to canned green peas during end-over-end sterilisation

The effect of temperature (110, 120 and 130 °C), rotation speed (5, 10 and 15 r.p.m.) and headspace (4, 8 and 12 mm) on heat transfer coefficients to canned green peas during end‐over‐end sterilisation was studied using response surface methodology. The models developed for fluid‐to‐particle heat transfer coefficient, h_fp, and overall heat transfer coefficient, U, were adequate, showing no significant lack of fit and satisfactory correlation coefficients. For the two responses, temperature, rotation speed and headspace have a significant effect. U, ranged between 477 and 905 W m^?2 °C^?1, while h_fp, fluctuated between 480 and 1950 W m^?2 °C^?1. The highest h_fp and U values are obtained at high temperatures, rotation speeds and headspaces. The verification of the prediction models was satisfactory. Dimensionless correlations were developed for h_fp and U, with equations showing a good agreement with the experimental data. Heat transfer to liquids and particles was modelled using the Reynolds number, the Prandtl number and adimensional headspace.     

Heat transfer during contact cooking of minced meat patties

Experimental results on heat transfer during heating of four types of minced meat patties by contact with hot plates are given. Experimental equipment, method of measurement and experimental values of heat flux from heating plate to heated product surface during 4 min contact cooking are described. Simultaneous measurements of product and heating plate temperatures as a function of time were used to determine the contact heat transfer coefficient between heating plate and product surface. The results show that the heat flux varied during the heating process. The magnitude of the heat flux as a function of time varies with the type of material heated and the heating plate temperature. The heat transfer coefficients measured were in the range 200 to 1200 W m^?2 K^?1, depending on product type, contact plate temperature, contact pressure and stage in the heat treatment.     

Thermophysical Properties of Apples in Relation to Freezing

The temperature dependence of different thermo-physical properties of Golden Delicious and Granny Smith apples were studied. Regression relationships for apparent specific heat, thermal conductivity and thermal diffusivity, density changes in the unfrozen and frozen states for the two varieties of apples and the surface heat transfer coefficients associated with freezing under different conditions were discussed. The mean values of the different properties for Golden Delicious apples were (values in the parentheses are for Granny Smith apples): thermal conductivity 0.427 and 1.45 W/m²C° (0.398 and 1.22 W/n²C°); apparent specific heat, 3.69 and 1.95 kJ/kgC° (3.58 and 1.68 kJ/kgC°); and density, 845 and 788 kg/m³ (829 and 786 kg/m³), respectively, in the unfrozen and frozen states. The surface heat transfer coefficients, as determined by using Plank's equation, ranged from 12.7 W/m²C° for freezing in air to 68.4 W/m²C° for freezing in liquid nitrogen.     

Heat Transfer Coefficients to Liquids with Food Particles in Axially Rotating Cans

Heat transfer rates were measured for steam-heated, rotating cans containing potato spheres in water. Can rotational speed (9.3–101 rpm), sphere size (22.2–35.0 mm), and potato volume fraction (0.107–0.506) were varied in 14 triplicated experiments. Overall heat transfer coefficients were correlated within ±25% with physical properties and operating variables by an equation derived by dimensional analysis. Film coefficients (h_p) for heat transfer from the water to potatoes in the can were determined by thermocouple measurement of potato surface temperatures; values were found to be finite and nearly invariant, averaging h_p= 160 ± 30 W/m²K. The lack of variation of h_p suggested that for the experimental conditions tested, there was little relative motion between liquid and particles.     

An inverse lumped capacitance method for determination of heat transfer coefficients for industrial air blast chillers

An inverse model using lump body was developed to calculate heat transfer coefficients as a dynamic function of time. The model used sequential function specification algorithm to calculate surface heat flux from transient temperature measurements inside a lump body system. Transient temperature measurements were collected during cooling inside an industrial chiller at two different positions with different air velocities using four replicates for each position. The calculated surface heat flux was found to be very accurate as the maximum value of the root mean squares error (RMSE) for temperature is 0.045 °C, lower than the expected error form thermocouple measurements. The calculated heat flux was then used to calculate heat transfer coefficients as a dynamic function of cooling time followed by calculation of time average heat transfer coefficient using numerical integration. The approach developed here could be a pragmatic powerful dynamic method to model spatial variation of heat transfer coefficients for industrial chillers and freezers.     

THERMAL TRANSPORT IN A MULTIPLE JET IMPINGEMENT OVEN

Three‐dimensional numerical simulation of flow field and heat transfer in a jet impingement oven with multiple jets was carried out. Distribution of local heat transfer coefficient on a model cookie in the oven cavity was obtained from the predicted flow and temperature fields, and compared with the experimentally measured values. Effects of air temperature (400, 450K) and air velocity (2.5, 5, 10 m/s), on the surface heat transfer coefficient were studied. Numerical predictions showed good agreement with the experimental results. It was found that heat transfer coefficient was a strong function of air velocity while air temperature had no effect on the heat transfer coefficient. Local heat transfer coefficient values ranged between 11 and 66 W/m²K whereas average heat transfer coefficient values ranged between 24 and 42 W/m²K. Results also showed that because of the presence of the suction fan in the oven, flow field near the model cookie surface was quite different from that for a jet impinging on a flat surface.     

HEAT TRANSFER TO WATER AND SOME HIGHLY VISCOUS FOOD SYSTEMS IN A WATER-COOLED SCRAPED SURFACE HEAT EXCHANGER

Heat transfer in a water-cooled scraped surface heat exchanger has been investigated. The overall heat transfer coefficient in the heat exchanger is composed of three elements: heat transfer coefficient in the coolant jacket, resistance to heat flow in the separation wall and heat transfer coefficient inside the scraped cylinder. A method for assessing the heat transfer coefficient at the coolant side was developed. In contrast with studies published elsewhere, heat transfer was investigated with food systems which are non-newtonian and possess a complicated and unknown flowing behavior at higher shear rates. For water and three starch-based food products (starch content 12–18%) the heat transfer coefficients inside the scraped cylinder were measured for shaft speeds ranging from 1.67 to 10 revolutions/s. The experimental results were compared with heat transfer coefficients calculated with a model based on the penetration theory. For the starch-based products, in general, no consistent interactions between mass flow rates and internal heat transfer coefficients were observed. In the shaft speed range studied heat transfer coefficients at scraped surface varied from 3200 to 7800 W/m² K for water, from 500 to 3150 W/m² K for velouté sauce, from 670 to 1330 W/m² K for roux and from 780 to 1900 W/m² K for ragout.     

CONJUGATE HEAT TRANSFER ASSOCIATED WITH A TURBULENT HOT AIR JET IMPINGING ON A CYLINDRICAL OBJECT

Numerical simulation of flow field and thermal transport processes for a turbulent jet impinging on the surface of a cylindrical object, e.g., hot dog, were carried out at different jet velocities. Numerical predictions for average surface heat transfer coefficient were compared with the experimental results obtained using a lumped mass approach for model aluminum cylinders. Results indicated good agreement between the numerical and experimental results over the range of jet velocities, i.e., 10–40 m/s. Further experiments were carried out with real beef hotdogs to obtain center point temperature–time history during hot air impingement baking process. The experimental data agreed well with the numerical predictions when the impinging jet temperature was below 100C.     

Effects of Heat Transfer by Radiation and Convection on Browning of Cookies at Baking

The effects of modes of heat transfer (radiation or convection) on the baking color development of food were studied. An experimental baking oven that could be altered to two heat transfer modes was designed; the ratio of heat by radiative transfer to total heat transferred was about 30% or 70%. The glucose-glutamate solutions were heated at different air temperatures to measure the browning rates to calculate the activation energies. Cookies were baked at 200°C to measure the lightness of color on the surface and the surface temperature. It was clarified that the development of color depended on the temperature only.     

Modeling Heat Transfer During Oven Roasting of Unstuffed Turkeys

A finite element method was used to solve the unsteady state heat transfer equations for heating of turkeys in a conventional electric oven. Breast, and thigh and wing joint temperature in 5.9, 6.8, 8.6, 9.5, and 10.4 kg turkeys were simulated. A surface heat transfer coefficient of 19.252 W/m2K determined by transient temperature measurements in the same oven, was used. Thermal conductivity measured using a line heat source probe from 0 to 80°C was 0.464 W/mK. Simulated temperatures were within 1.33, 1.47, and 1.22°C of experimental values of temperature in the breast, thigh, and wing joint, respectively. Initial temperature 1,2, and 3°C lower than 4°C required additional baking time of 16,22, and 27 min., respectively for the thigh joint to reach the target endpoint temperature.     

Effect of nano silver coating on thermal protective performance of firefighter protective clothing

The main aim of this experimental work is to find out possible improvement in thermal protective performance of firefighter protective clothing when subjected to different level of radiant heat flux density. Firefighter protective clothing normally consists of three layers: outer shell, moisture barrier and thermal liner. When thermal protective performance of firefighter protective clothing is enhanced, the time of exposure against radiant heat flux is increased, which will provide extra amount of time to firefighter to carry on their work without suffering from severe skin burn injuries. In this study, the exterior side of outer shell was coated with nano-silver metallic particle through magnetron sputtering technology. Coating of outer shell with nano-silver particles was performed at three level of thickness, i.e. 1, 2 and 3?µm, respectively. All the uncoated and silver coated specimens were then characterized on air permeability tester, Permetest and radiant heat transmission machine. It was observed that coating has insignificant difference on the air and water vapor permeability of specimen and a significant decline was recorded for the value of transmitted heat flux density Q_c (kW/m²) and percentage transmission factor (%TF Q_o) as compared to uncoated specimen when subjected to 10 kW/m² and 20?kW/m² indicating improvement of thermal protective performance. These values go on further reduction with increase in thickness of coating layer of nano-silver particles.     

Modeling the Thermal Properties of a Cup Cake During Baking

Thermal properties of a cup cake were estimated under conditions simulating industrial baking. Densities (p: 803-236 kg/ m³), specific heats (C_p: 2516-2658 J/kgK) and thermal conductivities (k: 0.1064-0.2064 W/mK) were estimated using a pycnometric/geometric cutting method, a modulated differential scanning calorimeter and a line heat source probe. Cp and k were based on internal temperatures after specific baking times. Thermal diffusivities (a: 1.02×10^?7-1.698×10^?7m/s²) were obtained by dividing the thermal conductivities by the product of specific heats and densities. Based on thermal property data, simple empirical models were developed for further prediction.     

Water Velocity Effect on Heat Penetration Parameters During Institutional Size Retort Pouch Processing

Institutional size retort pouches (15 × 12 × 1) filled with 10% bentonite were processed in water with overriding air pressure. The heat penetration parameter, f_h, was measured at seven flow rates from 10 gal/min (Re = 3000) to 110 gal/min (Re = 33000). Apparent convection heat transfer coefficients (h-values) were calculated. Significant differences were found for both the h-value and observed f_h value as a function of flow rate. The h-values ranged from 33–48 BTU/hr ft² F and the observed f_h values ranged from 23.0–20.1 min for 10 and 110 gal/min, respectively.     

Computational fluid dynamics (CFD) modeling of an electrical heating oven for bread-baking process

Radiation is the most dominant heat transfer mode in an electrical heating oven. A 3D CFD model for an electric heating baking oven was developed. Three different radiation models namely, discrete transfer radiation model (DTRM), surface to surface (S2S) and discrete ordinates (DO) were employed for the simulation of the electrical baking oven. All models predicted almost similar results, which tallied well with the experimental measurements. During the full heating cycle, the oven set-point temperature was reached after 360 s. Lower temperature zones occurred near oven wall due to lower air flow. Based on preliminary evaluation of applicability, the DO radiation model was selected for bread baking simulation and validated with the experimental measurement of bread temperature. Bread simulation was carried out to study the profiles of temperature and starch gelatinization of crust and crumb of the product. This study indicated the baking process to be complete at 1500 s when the temperature of bread-center reached 100 °C.     

HEAT TRANSFER COEFFICIENT FOR COOKIE SHAPED OBJECTS IN A HOT AIR JET IMPINGEMENT OVEN

Correlations for average heat transfer coefficient for cookie shaped objects in a hot air jet impingement oven were obtained using aluminum cookie models. The study was carried out in a pilot plant scale hot air jet impingement oven. The effects of individual cookie position, presence of surrounding cookies, air velocity, air temperature and rotation of the oven plate on average surface heat transfer coefficient were investigated. The value of the heat transfer coefficient ranged between 100–225 W/m²K and was found to be a strong function of jet air velocity. The impact of surrounding cookies on the heat transfer coefficient was more for smaller cookies, which had larger cookie‐to‐cookie spacing.     

Numerical simulation of heat transfer in protective clothing with various heat exposure distances

This paper reports on a model of heat transfer through multi-layer firefighter protective clothing with an air gap under radiant heat flux of 8.5 kW/m². The model considers the dynamical changes of heat exposure distance due to human body movement. The predictive results were found in good agreement with the experimental measurements. Numerical model was employed to study the effects of heat exposure distance and moving speed on heat transfer in multi-layer fabric system. The results showed that the heat exposure distance had an significant impact on skin burn. The safe zones for firefighting operation were more than 0.3 m for 2nd degree burn and 0.15 m for 3rd degree burn when the fabric system was exposed to 8.5 kW/m² for 300 s. However, the moving speed could speed up the time to 2nd degree burn but alleviate the time to 3rd degree burn.     

Convection and radiation combined surface heat transfer coefficient in baking ovens

The combined surface heat transfer coefficient is a determining parameter of convective baking process time and efficiency, as well as the resulting food product quality. By this study, the combined surface heat transfer coefficient term was determined at the convective oven temperature range of 70–220 °C, with fan (turbo) and without fan (static oven) applications. The methods of “Lumped Capacity” and “Time–Temperature Matching” were used. Both methods utilize the time–temperature data at a fixed position of a definite material, during unsteady state heating up period inside the convective oven. The increase in oven temperature and the fan application in the oven derived higher calculated values of surface heat transfer coefficients. Good agreement was observed between both methods and the literature values. The given methods are applicable to other oven types and heating modes.